Senichi Hoshino announced he will step down as manager of the Tohoku Rakuten Eagles at the end of the season during a press conference in Sendai on Thursday.
He guided the Eagles to their first Japan Series title last year, but the club began play on Thursday in last place in the Pacific League.
Hoshino - who suffers from ossified yellow ligaments and was forced to take leave in late May, and undergo surgery in mid-June - said time he missed with the club for treatment as well as its place in the PL standings are why he is calling it quits.
"The main reason is because of our [poor] performance, and also because I had to spend two months away from the team," said the 67-year-old.
Eagles owner Hiroshi Mikitani, who sat alongside Hoshino at the press conference, urged the skipper to stay on, but his mind was made up.
"As a competitor, having to step away from the front lines during the season is something that should never happen, and that's why I made the decision."
His ailment occurs when the ligaments that run along the spine thicken and harden, putting pressure on the spinal cord. Hoshino returned to the bench in late July.
Hoshino, who managed the Chunichi Dragons before going to the Hanshin Tigers, accepted the Rakuten job in 2010.